1. Field of the Invention
The invention relates to a new process for the asymmetric hydrogenation of the dihydrofuroimidazole derivatives of the general formula: ##STR3## in which R.sub.1 means a protective group which is cleavable in a known way and R.sub.2 stands for hydrogen or a protective group which is cleavable in a known way, with hydrogen in the presence of a homogeneous catalyst to the corresponding diastereomeric tetrahydrofuroimidazole derivatives of general formula: ##STR4## in which R.sub.1 and R.sub.2 have the above-mentioned meanings.
2. Background Art
Most known (+) biotin syntheses pursue the aim of dividing suitable precursors by, for the most part, very costly methods of racemate resolution with, for the most part, very expensive cleaving agents and further pursue the (+)-biotin synthesis with the resulting diastereomers (see, e.g., German Patent No. 2,058,248). Then, according to European Published Patent Application No. 273,270, the introduction of the relevant optically active centers, meaning the 3aS and 6aR positions of the biotin ring structure, was achieved for the first time by an asymmetric hydrogenation of the corresponding dihydrofuroimidazole derivatives with a typical hydrogenation catalyst, such as, rhodium on aluminum oxide. This process was not completely satisfactory because of the attainable yield of the desired diastereomer.